Added Google Colab ability for .ipynb files

Author: rickecon

docs/book/_config.yml

@@ -66,7 +66,7 @@ launch_buttons:
   # binderhub_url          : https://mybinder.org  # The URL of the BinderHub (e.g., https://mybinder.org)
   # jupyterhub_url         : ""  # The URL of the JupyterHub (e.g., https://datahub.berkeley.edu)
   # thebelab               : false  # Add a thebelab button to pages (requires the repository to run on Binder)
-  colab_url          : "" # The URL of Google Colab (https://colab.research.google.com)
+  colab_url: "https://colab.research.google.com"  # The URL of Google Colab
 
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 # FiscalSim-US repository settings